This invention relates to deflection yokes including a beam positioning magnet arrangement.
Multiple beam color picture tubes such as used in color television receivers generally require some arrangement for maintaining convergence of the three beams as they are deflected to form a raster to reproduce a color image on the viewing screen or to correct for various types of raster distortion such as pincushion distortion.
Picture tubes of the delta-gun type generally utilize a convergence assembly mounted on the neck of the picture tube with permanent magnets and dynamically energized electromagnets to maintain the desired beam convergence. Picture tubes of the in-line gun type utilize deflection yokes of the self-converging type in which the nonuniformity of the deflection fields is selected to maintain beam convergence. For various reasons such as the use of wide-deflection angle picture tubes and the use of different types of deflection yokes it has been necessary or desirable to supplement the above-described convergence arrangements with further apparatus. To this end it is known that permanent magnets can be appropriately located to modify the beam deflection fields to achieve the desired beam convergence or raster geometry.
As an aid to influencing one or more beams, perhaps at a particular portion or portions of the scanned raster, it is desirable to provide adjustment of the supplemental magnetic field direction and intensity. The former can be achieved by rotating the magnet poles. The latter has been achieved by substituting a different strength magnet or by rotating the poles of one magnet relative to the poles of another closely spaced magnet. When like poles overlap the greatest intensity field is produced; when opposite poles overlap the least intensity field is produced. A main problem with the rotatable magnet intensity control approach is that because the centers of the magnets are spaced from each other the fields cannot be cancelled at all points when opposite poles overlap. In the case of two equal intensity magnets the fields cancel only at a point midway between the magnets and are of opposite polarity in opposite directions away from the center. In the case of two unequal strength magnets with opposite poles overlapping the fields will cancel at two points away from the center distance between the magnets with net opposite polarity fields in opposite directions from each point.
When the poles of the two magnets are not directly opposed the general effect described above becomes much more complex, but, in general, it can be stated that the resultant field will change in intensity and polarity as a function of distance from the magnets. Obviously, these conditions make it very difficult, if not impossible, to trim exactly the main deflection field to achieve the desired beam or raster correction.